As methods for obtaining an optically active amine compound, classical methods have been often used, such as a method in which a naturally-derived amine compound is used, and a method in which a racemic amine compound is synthesized, and then subjected to optical resolution using an optically active organic acid. However, the former method is still disadvantageous in that the raw material substance is difficult to obtain. The latter method is still disadvantageous in that, although the optical purity of the amine compound can be improved by the recrystallization method, the amount of the desired compound cannot exceed the amount present before the optical resolution.
To solve these problems, the synthetic methods based on catalytic asymmetric hydrogenation reactions of prochiral carbon-nitrogen double bonds have been actively studied recently. In recent reviews, methods using complexes having rhodium, iridium, ruthenium, palladium, or titanium as the central metal are described (see Acc. Chem. Res., 2007, vol. 40, p. 1357, Organic Reactions, 2009, vol. 74, p. 1, and Chemical Review, 2012, vol. 112, p. 2557). In these reviews, cases in which rhodium metal or iridium metal, which are expensive, is used are abundant, and, reportedly, optically active amine compounds can be obtained in high yields and with high stereoselectivities. However, it is stated that, since the use of ruthenium metal, which is relatively inexpensive, may result in the loss of the activity of the ruthenium complex due to the produced amine compound in some cases, the application of ruthenium metal to the asymmetric hydrogenation reaction of a carbon-nitrogen double bond is limited, in comparison with the application thereof to the asymmetric hydrogenation reaction of a carbon-oxygen double bond which is applicable to wide varieties of substrates (see Organic Reactions, 2009, vol. 74, p. 1).
Examples of the catalytic asymmetric hydrogenation reaction of a carbon-nitrogen double bond using a ruthenium complex and hydrogen gas are described in the following documents. As for asymmetric hydrogenation reactions of various prochiral imines using RuCl2 (diphosphine) (diamine) and abase, N-(phenylethylidene)aniline, which is an acyclic imine, was asymmetrically hydrogenated with 92% ee at the highest, but the optical purity of 2-methylquinoxaline, which is a cyclic imine, dropped to 69% ee (WO2002/008169 or Adv. Synth. Catal., 2003, vol. 345, p. 195). As for an asymmetric hydrogenation reaction of 2-methylquinoxaline using RuCl2[(S)-hexaphemp][(S,S)-dach], only an amine with a moderate optical purity of 81% ee at the highest was obtained (WO2001/094359). As for an asymmetric hydrogenation of 2-methylquinoxaline using Ru(p-cymene)(monosulfonylated diamine) (BArF), an amine with a high optical purity of 98% ee was obtained when the amount of the catalyst was 1 mol %, but the optical purity dropped to 93% ee, when the amount of the catalyst was reduced to 0.1 mol % (Org. Lett., 2011, vol. 13, p. 6568).
Meanwhile, there is a report on a case of an asymmetric hydrogenation reaction of 3-substituted-2H-1,4-benzoxazine using an Ir-diphosphine complex, but there is no report on a case of using Ru metal, which is relatively inexpensive (Adv. Synth. Catal., 2012, vol. 354, p. 483).